A High Selectivity Crystal Set

This & other outdoor photo by Daun Yeagley at MVUS picnic Aug. 3, 2002
(All other photos by Dave Lundy)
Over the years, Ed Garner WR8A, had told about how he always looked for KOA 850 Kc. from Denver on his crystal set. I thought, yeh, right... Everything I had ever built couldn't separate WHIO from WING which are both in Dayton, Ohio and they are 1200 Kc apart. My curiousity got the best of me so instead of me inviting myself over to see his set, I invited him over here to try an antenna /pa110047.jpg wire suspended from a helium filled baloon. Well, as soon as I saw his crystal set, a few thoughts came to mind immediately. He uses adjustable spacing between several "stages" to adjust the coupling between the stages to eliminate what I had always experienced, overloading! Not to mention huge coils wound of #8 copper wire and 8" in diameter. Ed uses 4 stages in his set. The first stage has the antenna wire coming in to the top of the coil, then through the tuning capacitor and then to ground, a series circuit. Then there are 2 intermediate stages which are just parallel tuned circuits. The detector is a random length of small gage wire with a germanium diode going the headphones and then back the the coil. Then we got to talking about Q and how this could be a large factor on selectivity. Ed told me the highest Q coils he had made were around 800. Well, let's see what I can do with all this new visual information.

Very shortly after this, I found a good source of 1/4" copper tubing and came home with 8 boxes of 50' lengths in each box. My immediate thoughts were how to make the "coils" and how to maintain good spacing.

After obtaining several large air variable capacitors and finding out that they were only 270 pf each, I had to make the coils a little larger in inductance. I found out that I needed about 120' of 1/4" tubing to resonate in the broadcast band with those capacitors. The next game to play was to determine the shape factor of the coil using one conductor diameter as the spacing. This will be 2 turns per inch and the shape factor will be 1 x 1 for this design.

There will be twice the number of turns as the diameter. For instance, if you choose a 12" diameter for your coil, it will be 19 turns long which would be a 1.6 x 1 ratio. That would not be unusable. However, a close formula would be L, 1440" , divided by the length in inches in one turn equals the number of turns divided by the spacing in inches. Also, figure an even number of turns and allow 1 foot on each end to be bent up to reach the tuning capacitor. In my case, I made a 15" diameter form 16" long.

pc210340.jpgTo make the coil form, I cut out 2 discs of 3/4" plywood 11 1/2" diameter. Locating the center of the discs, I drilled 1 1/4" holes to accept a piece of pipe. I cut out 36 - 15/16" wide pine strips 18" long to be attached around the circumference of the plywood discs. Leave 3 strips removable at the 12, 3, 6, and 9 o'clock positions. Also, leave 50% of the strips removable so the coil can be removed from the form. All the rest of the strips can be nailed on.

pc210338.jpg pc210339.jpg To make the spacing as uniform as possible, I drilled a pair of strips, 1/2" x 3/4" x 18" long together on the vertical mill using a 1/4" drill bit. I made 4 sets of spacer sticks. Wind up the tubing as best you can wearing gloves to keep finger prints off the copper. Remove the sticks at the 12 o'clock position to install the spacer sticks and the clamp for gluing. The clamp was made out of two pieces 3/16" thick 2" wide aluminum plates 20" long. The hole pattern was drilled on the mill, also. pc210348.jpg pc210345.jpg pc210342.jpg I drilled through both pieces using a .159 drill for a 10 x 32 thread. Drill the other plate holes out with a .190 drill to "clear" a 10 x 32 screw. I drilled the pattern so I could use screws between every turn of the coil, but after using this clamp, every third turn is fine. It can squash the pine spacer sticks very easily! I used Elmers water soluable wood glue for the project. If some aspect of your work has to be re-done, simply soak the coil in the bathtub over night and the whole thing will fall apart and all the parts are re-usable! (Don't tell the wife) This coil has a Q of 1040.

pc210349.jpg pc210360.jpg pc210361.jpg After the coil has been built, I made 10" x 20" plywood pieces to hold the coils and capacitors. /pa110046.jpg The dial plates are 1/8" white plexiglass and the pointers are 1/4" thick 10" long clear plexiglass. The knobs are 1 1/4" diameter Delrin. The 2" x 4" x 8' frame has a notch cut out of one edge for the plywood pieces to slide to adjust the coupling between the stages. The 8' long frame is NOT long enough for proper spacing for strong stations for the 3 stages, which I use.

pc210353.jpgTo verify the possibility of reaching the point of diminishing returns, I made another coil 18" diameter 18" long using 1/2" diameter copper tubing. Still a 1 x 1 ratio and the measured Q nose dived down to 680. In actual use, I CAN tell the difference without using an instrument to tell me the difference! This coil was made before I made the clamp. See all those ugly screw holes....

I live near Dayton, Ohio and this set allows me to listen to WOR 710 Kc in New York with WLW 700 Kc in Cincinnati right next to them! By the way, I use Baldwin headphones. So, high Q coils and proper interstage coupling is the answer.

xtal-set-schematic.gifHere is the schematic.

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